In piston-type internal-combustion engines, whether they are externally ignited or operate with a self-ignition of the fuel mixture, the main requirements are high specific performance, low pollutant emission and low fuel consumption. A shift of the operating point towards the higher loads, accompanied by a simultaneous reduction of the stroke volume ("downsizing") was found to assist in lowering the fuel consumption in the partial load run of piston-type internal-combustion engines.
Externally ignited piston-type internal-combustion engines have been provided with a charging device, preferably an exhaust gas turbocharger for obtaining uniform torque and output values despite a smaller stroke volume as compared to conventional engines. In such an engine the tendency of a disadvantageous knocking increases with increasing charging grade and thus with increasing terminal compression pressures and terminal temperatures. For this purpose the turbocharged engines conventionally are driven with a smaller compression ratio than comparable, naturally aspirating engines. A lower compression ratio, however, adversely affects the degree of efficiency of the thermodynamic process.
To remedy the above-discussed problem, internal-combustion engines have been constructed which have a shiftable crankshaft for varying the compression ratio. Such an arrangement permits the engine to run in partial load conditions with a high compression ratio and a lean mixture and in full load conditions with a stoichometric mixture and a low compression ratio.
To be able to use conventional catalysts in internal-combustion engines of the above-outlined type, during the leaner operation the calibration of the ignition moment/air ratio has to be selected such that only a small nitrogen oxide (NO) emission results. This, however, is conditioned on a combustion process with a superior lean run capability.
As concerns thermodynamic considerations, a lean operational mode and a high compression ratio is advantageous in the full load condition as well. For this purpose the engine has to be designed such that it has a sufficient lean-run capability to ensure that the raw NO emission is limited and that the operation is free from misses at a sufficient distance from the lean-run boundary. Further, in externally ignited engines the tendency to knock has to be reduced by means of the usual concepts.
The known combustion chamber designs required for such a purpose in lean-running engines are borrowed either from four-valve piston engine designs which are generally used in personal automobiles and which are provided with "tumble" intake ports to ensure the desired turbulence by the time combustion starts, or they are based on two-valve piston engines which operate with an "intake twist". In either case, the outer face of the piston crown which essentially determines the shape of the combustion chamber is as "smooth" as possible, that is, it is free from discontinuities (breaks, fissures, etc. in the smooth surface), because such discontinuities are considered to disadvantageously affect the combustion process.
In piston-type internal-combustion engines which operate on self-ignition, for example, diesel engines with direct fuel injection, a high turbulence is desirable to reduce the emission of carbon black (soot).